1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus, in which the refractive index of a sub beam incident on a light receiving element is adjusted to ensure that the sub beam does not incide on a position deviated from the light receiving element, whereby information can be recorded and/or reproduced onto and/or from discs in various specifications.
2. Description of the Related Art
In general, an optical pickup apparatus is employed in CDPs, DVDPs, CD-ROM drivers, and DVD-ROM drivers, to record and/or reproduce information onto and/or from a non-contact type recording medium.
The DVDP (Digital Versatile Disc Player) and the DVD-ROM are capable of recording and reproducing high-density data so that they are preferably used with video and audio products. In particular, the optical pickup apparatus for use in the DVDP is capable of recording and/or reproducing information on and/or from a DVD and is manufactured to be compatible with a CD family media such as CD, CD-R (Recordable), CD-I (Interactive), and CD-G (Graphics).
However, the thickness of DVDs has been standardized differently from that of the CD family media in consideration of the allowable error in the tilt angle of a disc and the numeral aperture (NA) of an objective lens. That is, the standard thickness of existing CD family media is 1.2 mm, whereas the standard thickness of DVDs is 0.6 mm. As such, when an optical pickup apparatus for DVDs is used for the CD family media, spherical aberrations occur due to the difference in the thickness between these two types of discs. As a result of the spherical aberrations, a sufficient light intensity cannot be provided for recording an information signal or the signal reproduced from the CD is often deteriorated. In addition, the track pitch of a DVD is at least two times denser than a CD, and the space between each land is much narrower, so that the DVD storage capacity is several times larger than the CD storage capacity. Because a CD and a DVD have different thicknesses and storage capacities from each other, wavelengths of laser beams used for the DVD are different from those used for the CD. For instance, the CD uses an infrared ray having a wavelength of 780 nm for recording and/or reproducing information, while the DVD uses a red light beam having a wavelength of 635 nm for the same operation.
As described above, wavelengths of lasers used for the DVD and the CD are different because the DVD and the CD have different thicknesses. Therefore, it is difficult to reproduce information from the DVD and the CD using one optical system. As an attempt to solve this problem, a lens switching method has been developed. According to the lens switching method, two objective lenses, one for the CD and the other for the DVD, are installed. Even though it makes the product design more complicated and increases the cost of manufacture, it is the best known method for use with the DVD and the CD. In effect, Toshiba and Hitachi are still using this method in their products.
Then, there was introduced a new method for reading information from two media at the same time using only one DVD lens. An NA control system and an annular system are typical examples thereof. Since these two systems use only one lens, they are cost effective and simplify the product design. Furthermore, a hologram using two focal points with one DVD lens, or a non-spherical lens corresponding to the NAs of two media can be used also.
Meanwhile, optical pickup apparatuses, depending on the type of a light receiving element used, are divided into optical pickup apparatuses based on a push-pull method using a 4-split light receiving element, and optical pickup apparatuses based on a 3-beam method using an 8-split light receiving element. The 3-beam optical pickup apparatus is generally used for high-density discs where an accurate tracking servo performance is highly desired.
As can be seen in FIG. 1, the 3-beam optical pickup apparatus includes an LD (Laser Diode) 101 for emitting two light beams of different wavelengths; a diffraction grating 102 for splitting a laser beam from the LD 101 into three discrete beams, namely one main beam and two sub beams; a beam splitter 103 for reflecting a beam off of the diffraction grating 102 towards an optical disc and transmitting a reflected beam from the optical disc; a collimator lens 104 for condensing parallel beams reflected from the beam splitter 103 to form a beam spot on an information recording surface of the optical disc, and transforming a divergent beam of the beam spot reflected from the information recording surface of the optical disc to a parallel beam (or collimated beam); an objective lens 105 for condensing the reflected light transmitted through the beam splitter 103, and a photo detector 106 for photoelectrically transforming three beam spots condensed by the objective lens 105 into electric signals.
Depending on whether the information recording medium is a CD or a DVD, the LD 101 of the optical pick up apparatus outputs laser beams having different wavelengths. An outputted laser beam is then split by the diffraction grating 102 into a main beam and sub beams. In other words, the diffraction grating 102 splits the laser beam into an 0th-order main beam for reading an information signal from the laser beam outputted from the LD 101 and for a focus servo of the optical disc and ±1st-order sub beams for a track servo of the optical disc.
The split beams are reflected towards the optical disc by the beam splitter 103, and focused onto the optical disc by the collimator lens 104 to form beam spots, and reflected again from the optical disc. The reflected beams from optical disc transmit through the beam splitter 103, and are collected by the objective lens 105 and detected by the photo detector 106.
As illustrated in FIG. 2, an 8-split light receiving element composed of eight discrete photoelectric terminals A, B, C, D, E1, E2, F1, F2 is used as the photo detector 106. Four split terminals at the center A, B, C, D belongs to a main beam spot portion 106a receiving a reflected ray of the main beam, and the other four split terminals E1, E2, F1, F2 on both sides of the main beam spot portion 106a belong to a sub beam spot portion 106b. The number and the position of the terminals of the main beam spot portion 106a and the sub beam spot portion 106b can be varied depending on a desired product design.
The photo detector 106 detects a main beam that is focused on the main beam spot portion 106a, and reads information stored in the beam and detects a focusing signal error of the optical disc. Further, the photo detector 106 detects a tracking signal error of the optical disc using the sub beam focused on the sub beam spot portion 106b. 
However, laser wavelengths outputted from the LD 101 are different depending on which kind of discs is used. If the optical system is adjusted to one of those wavelengths, the sub beam is not properly incident on the sub beam spot portion 106b of the photo detector 106 as shown in FIG. 3. To prevent this, the optical pickup apparatus is now designed to have the diffraction angle or the rotation angle of the diffraction grating 102 change adaptively to the kind of disc used. When the rotation angle of the diffraction grating 102 is changed, the deviation of the sub beam from its incidence path to the photo detector 106 is negligible because the range of the rotation angle is not greater than 5 degrees. However, when the diffraction angle is changed, the degree of deviation of the sub beam varies according to the wavelengths of the lasers used. In order to receive incident sub beams deviated from their incidence path, some suggested to form a plurality of sub beam spot portions 106b at every place the sub beams incide. However, this only complicated the structure of the photo detector 106. In this case, as shown in FIG. 3, more than one sub beam having different wavelengths overlap with each other. Thus, it is very hard to determine proper placement for the sub beam spot portions 106b to receive all sub beams having different wavelengths.
Even though there are optical pickup apparatuses compatible with DVDs and CDs, they do have several problems to be solved. Recent technical advances in the compact disc industry have brought a BD (Blue ray Disc) with a capacity of 25 G bytes, and there is a very high possibility that new discs with different specifications and greater capacities may be developed in the near future. Unfortunately, the aforementioned methods are for CDs and DVDs with existing specifications. Accordingly, there stands a need to redesign an optical system for reproducing information from those discs of newly developed specifications, or to develop a new optical system for reproducing information from those newly developed discs.
As such, there is a need to develop an optical pickup apparatus compatible with CDs and DVDs and other discs having different specifications that might be developed in the near future.